1. Field of the Invention
The present invention relates to a method of forming a polysilicon film, and more particularly, to a method of forming a polysilicon film of thin film transistors of thin film transistor liquid crystal display (TFT-LCD).
2. Description of the Related Art
Generally, an active matrix liquid crystal display may be classified based on material used for fabricating thin film transistors, namely a polysilicon thin film transistor display and an amorphous silicon thin film transistor display. Because the polysilicon thin film transistor can integrate driving circuits, it can provide a higher yield and fabrication cost thereof is lower than the amorphous silicon thin film transistor. In addition, a polysilicon film can provide better electrical characteristics than an amorphous silicon layer, and can be used on a glass substrate to fabricate electronic devices. Another advantage of the polysilicon film transistor technology is its higher resolution capability so that the size of the devices fabricated using the polysilicon film transistor technology can be substantially reduced. General mass-produced polysilicon thin film transistor displays can be fabricated by utilizing low temperature fabricating technology (at a range of about 450° C. to 550° C.), for example, a low temperature thin film process for forming a high quality gate dielectric layer and ion implantation process for large size substrates.
Because of cost concern of glass substrates, a solid phase crystallization (SPC) process is applied for forming a thin film under low temperature. However, the process temperature of about 600° C. is too high and therefore adversely affecting crystallization of the thin film. Alternatively an excimer laser is applied in an excimer laser crystallization (ELC) process or excimer laser annealing (ELA) process for low-temperature thin film crystallization, wherein the laser scans and melt an amorphous silicon layer in order to crystallize and transform an amorphous silicon into a polysilicon film.
FIGS. 1A and 1B illustrates a sectional view illustrating a fabrication process of forming a polysilicon film in accordance with a prior art.
Referring to FIG. 1A, a substrate 100 is provided. An insulation layer 102 and an amorphous silicon layer 104 are sequentially formed on the substrate 100. A patterned anti-reflective layer 106 comprised of a silicon nitride layer is formed the amorphous silicon layer 104, and thereby defining a non-exposure region 130 (covered by the anti-reflective layer 106) and an exposure region 140 (not covered by the anti-reflective layer 106).
An excimer laser 108 having sufficient energy is then applied to the structure mentioned above. Because the anti-reflective layer 106 has a capability of enhancing the exposing efficiency of the excimer laser 108, and therefore the temperature of the amorphous silicon layer 104a in the non-exposure region 130 is higher than that of the amorphous silicon layer 104b in the exposure region 140. Therefore, the amorphous silicon layer 104a in the non-exposure region 130 is completely melted and the amorphous silicon layer 104b in the exposed region 140 is partially or incompletely melted.
Referring to FIG. 1B, the unmelted amorphous silicon layer 104b then serves as a nucleation site/discrete seed for re-crystallization. Therefore, the crystallization of the polysilicon layer is performed from the amorphous silicon layer 104b laterally towards the amorphous silicon layer 104a, i.e. the direction of arrow 110, thereby forming a polysilicon layer 112a and a polysilicon layer 112b. 
FIGS. 2A and 2B illustrates another fabrication process of forming a polysilicon film of a prior art.
Referring to FIG. 2A, a substrate 200 is provided. An insulation layer 202 and an amorphous silicon layer 204 are sequentially formed on the substrate 200. A patterned silicon nitride layer 206 is formed on the amorphous silicon layer 204 covering a portion 240a of the amorphous silicon layer 204 defining a non-exposure region 230. A portion 204b not covered by the amorphous silicon layer 204 remain exposed is defined as an exposure region 240. The silicon nitride 206 serves as a heat sink.
An excimer laser 208 is then applied to the above structure. Because the heat sink 206 is capable of reflecting the excimer laser 208, while the portion 204b of the amorphous silicon layer 204 in the exposure region 240 is exposed to the excimer laser 208, and therefore the temperature of the excimer laser is absorbed by the exposed portion 204b of the amorphous silicon layer 204. Thus, the temperature of the exposed portion 204b of the amorphous silicon layer 204 in the exposed region 240 is higher than that of the portion 204a of the amorphous silicon layer 204 in the non-exposure region 230. Therefore, the portion 204b of the amorphous silicon layer 204 in the non-exposure region 230 is substantially melted while the portion 204a of the amorphous silicon layer 204 in the non-exposure region 230 is partially or incompletely melted.
Referring to FIG. 2B, the un-melted portion of the portion 204a of the amorphous silicon layer 204 serves as a nucleation site/discrete seed for crystallization process. Therefore, the polysilicon layer 212a, 212b, is formed by crystallization of the portion 204a of the amorphous silicon layer 204 which occurs along a lateral direction starting from the portion 204a of the amorphous silicon layer 204 towards the portion 204b of the amorphous silicon layer 204. That is, along the direction of arrow 210 as shown in FIG. 2B.
It is to be understood that in both of the methods mentioned above, the temperature differences between the portions 104a/104b, 204a/204b of the amorphous silicon layer 104/204 vary and are limiting factors, and accordingly, the grain size of the crystallization will be affected and limited therefrom.